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The antenna model used for correcting the influence of the antenna pattern on synthetic aperture radar (SAR) images requires on-ground validation and in-flight verification. A methodology for the in-flight verification that is based upon the measurement of azimuth antenna patterns using ground receivers has been successfully demonstrated for the operational SAR modes of the TerraSAR-X (TSX) and TanDEM-X (TDX) missions. Recently, the novel (terrain observation by progressive scans) TOPS mode was for the first time implemented as an experimental mode on TerraSAR-X to demonstrate its feasibility in support of its implementation on ESA’s Sentinel-1 mission. In this mode, besides scanning in elevation, the antenna beam is steered in flight direction from aft to the front at a constant rate to achieve an enhanced radiometric image performance. This paper discusses the methodology and presents results of the first in-flight antenna characterization of a SAR instrument operating in TOPS mode, in this case TerraSAR-X, using ground receivers. The results demonstrate that the TOPS one-way azimuth antenna pattern can be accurately modeled by the TSX antenna model indicating the general suitability of this approach for the in-flight antenna model verification during TOPS mode operations. [ABSTRACT FROM AUTHOR]